Conventionally, the 3rd Generation Partnership Project (3GPP) has studied evolution (hereinafter, referred to as “Long Term Evolution (LTE)” or “Evolved Universal Terrestrial Radio Access (EUTRA)”) of radio access scheme and radio networks of cellular mobile communication, and radio access scheme and radio networks (hereinafter, referred to as “Long Term Evolution-Advanced (LTE-A)” or “Advanced Evolved Universal Terrestrial Radio Access (A-EUTRA)”) for actualizing faster data communications using frequency bands wider than LTE.
In LTE, an Orthogonal Frequency Division Multiplexing (OFDM) scheme that is multicarrier transmission is used as a communication scheme of wireless communication (downlink) from a base station apparatus to a mobile station apparatus. Further, an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme that is single-carrier transmission is used as a communication scheme of wireless communication (uplink) from the mobile station apparatus to the base station apparatus. More specifically, a modulated transmission signal is transformed into the signal in the frequency domain by DFT (DiscreteFourierTransform), and the signal is mapped to radio resources (frequency resources) allocated by the base station apparatus, then is transformed into the signal in the time domain by IDFT (Inverse DFT) and is transmitted to the base station apparatus. In LTE-A, SC-FDMA is also referred to as DFT-precoded OFDM.
In LTE, in downlink are assigned the Synchronization Channel (SCH), Physical Broadcast Channel (PBCH), Physical Downlink Control Channel (PDCCH), Physical Downlink Shared Channel (PDSCH), Physical Multicast Channel (PMCH), Physical Control Format Indicator Channel (PCFICH), and Physical Hybrid automatic repeat request Indicator Channel (PHICH). Further, in uplink are assigned the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH), and Physical Random Access Channel (PRACH).
In LTE, a reference signal (Demodulation Reference signal; DMRS) used in demodulation of the PUSCH and PUCCH is time-multiplexed with the PUSCH or PUCCH and transmitted. The DMRS is subjected to code spreading using CAZAC (Constant Amplitude and Zero Auto-Correlation) sequences in radio resources divided with SC-FDMA assumed. The CAZAC sequences are sequences that have constant amplitude in the time domain and the frequency domain and that are excellent in auto-correlation characteristics. The sequences have constant amplitude in the time domain, and are thereby capable of controlling the PAPR (Peak to Average Power Ratio) to within low levels. Further, in the DMRS of LTE, by providing SC-FDMA symbols with the cyclic shift in the time domain, it is possible to perform CDM (Code Division Multiplex) on the DMRS spread using the same CAZAC sequence. However, when the sequence lengths of CAZAC sequences are different from one another, it is not possible to perform CDM. The generation method of DMRS in LTE is described in Non-patent Document 1, section 5.
Non-patent Document 2 proposes further applying orthogonal code (for example, Walsh-Hadamard Code [1,1] and [1,−1]) to DMRSs that are transmitted in different SC-FDMA symbols, in addition to above-mentioned CDM by cyclic shift, in order to enable uplink multi user spatial multiplexing (or also referred to as Uplink Multi User Multiple Input Multiple Output; UL MU-MIMO) between mobile station apparatuses assigned different radio resources in LTE. Hereinafter, the orthogonal code is referred to as orthogonal cover.